The disclosure of Japanese Patent Application No. 2003-028625 filed on Feb. 5, 2003, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a control apparatus and method for an internal combustion engine.
2. Description of the Related Art
An engine mounted in a vehicle such as an automobile has been known which is provided with a valve characteristic varying mechanism that varies a valve characteristic of at least one of an intake valve and an exhaust valve. In this engine, a valve overlap amount of the intake valve and the exhaust valve is adjusted by driving the valve characteristic varying mechanism based on the engine operating state, i.e., the engine speed and engine load, so that an internal EGR amount appropriate for that engine operating state is obtained. That is, by adjusting the valve overlap amount, the internal EGR amount is able to be adjusted, for example, to a value that gives priority to engine output or a value that gives priority to exhaust emissions and fuel efficiency, depending on the engine operating state.
Also, in a spark ignition engine mounted in a vehicle, the ignition timing is retard-corrected based on a correction amount which is increased or decreased depending on the presence or absence of knock. Retard-correcting the ignition timing inhibits an increase in the temperature of the combustion chamber, thereby suppressing knock. In this case, an increase in the combustion chamber temperature is able to be suppressed by retard-correcting the ignition timing because when the ignition timing is retarded, the timing at which the combustion of the mixture in the combustion chamber takes place is delayed, such that some of the mixture is expelled as exhaust into an exhaust passage while still at the high combustion temperature. As a result, the amount of heat transferred to the combustion chamber walls during combustion of the mixture is less by the amount of heat of the expelled mixture.
If the ignition timing is retard-corrected to suppress knock, however, the optimum value for the valve overlap amount (internal EGR amount) may decrease according to the amount of retard, depending on the operating state of the engine. This kind of change in the optimum value occurs, for example, when the ignition timing is retard-corrected in order to suppress knock when the engine is in an operating state where priority should be given to exhaust emissions and fuel efficiency, for example. In this operating state, the actual internal EGR amount is adjusted to the maximum value to improve exhaust emissions and fuel efficiency, but as the ignition timing is retarded this maximum value decreases. Therefore, the valve overlap amount, which is a parameter that controls the internal EGR amount, needs to be reduced so the optimum value for the valve overlap amount is reduced when there is a retard-correction of the ignition timing.
If the optimum value for the valve overlap amount were to be reduced in this way, the valve overlap amount would be higher than the optimum value. As a result, the amount of internal EGR in the engine would become excessive and adversely effect fuel efficiency and torque fluctuation. JP(A) 11-125126 proposes technology for reducing the valve overlap amount based on a correction amount used in the retard-correction of the ignition timing in order to suppress the adverse effects from this excessive amount of internal EGR. The amount of decrease in the valve overlap amount at this time is set to a value that differs for each operating range, defined according to engine speed and fuel injection quantity.
Reducing the valve overlap amount in this way makes it possible to suppress the adverse effects on fuel efficiency and torque fluctuation due to an excessive amount of internal EGR following a retard-correction of the ignition timing in order to suppress knock. However, when the ignition timing is retard-corrected only an amount corresponding to the correction amount in order to suppress knock, there may not be much change in the optimum value for the valve overlap amount depending on the operating state of the engine. If the valve overlap amount were to be reduced in this case, the amount of internal EGR would end up being reduced more than is necessary.
For example, when the engine is in an operating state where priority should be given to engine output, a value less than the value for obtaining the maximum amount of internal EGR for that engine operating state is set as the optimum value for the valve overlap amount. This is because the amount of (exhaust) gas which does not contribute to combustion, but which is in the combustion chamber during combustion, increases as the amount of internal EGR increases. So if the amount of internal EGR were to be set to the maximum value, there is a possibility that engine output would drop. Also, when the engine is in an operating state where priority should be given to engine output, in particular, when the throttle valve is close to fully open, priority is given to increasing the intake air amount as much as possible rather than to obtaining a given amount of internal EGR. Accordingly, a value with which the maximum amount of intake air can be obtained is set as the optimum value for the valve overlap amount. The optimum value for the valve overlap amount at this time is a value less than the value with which the maximum amount of internal EGR is obtained.
When the valve overlap amount is adjusted to the optimum value in this way, even if a retard-correction of the ignition timing is performed, the actual internal EGR amount is already a value less than the maximum value so the internal EGR amount will not become excessive with that retard-correction. Therefore, the valve overlap amount, which is a parameter that controls the internal EGR amount, does not need to be reduced so the optimum value for the valve overlap amount does not change following the retard-correction of the ignition timing.
In this case, if the valve overlap amount were to be reduced based on the correction amount used in the retard-correction of the ignition timing, the valve overlap amount would become much smaller than the optimum value. As a result, the amount of internal EGR would be reduced more than is necessary. In JP(A) 11-125126, the amount of decrease in the valve overlap amount at this time is set to a value that differs for each operating range. However, because the fact that the valve overlap amount is still reduced remains unchanged, the internal EGR amount would inevitably be reduced more than is necessary.
In view of the foregoing problems, this invention thus provides a control apparatus and method for an internal combustion engine, capable of suppressing an amount of internal EGR from becoming excessive, while suppressing a valve overlap amount from being reduced more than is necessary when a retard-correction of an ignition timing is performed to suppress knock.
A first aspect of the invention relates to a control apparatus for an internal combustion engine, which suppresses knock by retard-correcting an ignition timing based on a correction amount that is increased or decreased depending on the presence or absence of knock, while controlling a valve overlap amount of an intake valve and an exhaust valve to a value appropriate for the operating state of the engine. The control apparatus is provided with a guard controller which applies an upper limit guard to limit the valve overlap amount using an upper limit guard value according to the correction amount and the operating state of the engine.
Depending on the operating state of the engine, a value such as one prioritizing engine output or one prioritizing exhaust emissions and fuel efficiency is made the optimum value, and the valve overlap amount is then adjusted so as to become that optimum value. Therefore, depending on the operating state of the engine, when a retard-correction of the ignition timing of an amount corresponding to a correction amount for suppressing knock is performed, a case in which the optimum value for the valve overlap amount is greatly reduced following the retard-correction may occur, or a case in which that optimum value is not reduced much at all following the retard-correction may occur. According to this construction, the upper limit guard value used to limit the valve overlap amount is set according to the correction amount and the operating state of the engine. As a result, that upper limit guard value can be made a value able to restrict the valve overlap amount to a value equal to, or less than, a value at which the internal EGR amount will not become excessive when a retard-correction of the ignition timing is performed. If the engine operating state is such that the optimum value of the valve overlap amount is greatly reduced with a retard-correction of the ignition timing, in which case the valve overlap amount would be larger than the optimum value, the valve overlap amount would be larger than the upper limit guard value, set as described above, upon the retard-correction. One example of this kind of engine operating state is one in which exhaust emissions and fuel efficiency should be given priority. In this case, the upper limit guard is applied and uses the upper limit guard value to limit the valve overlap amount so as to suppress the internal EGR amount from becoming excessive. On the other hand, if the engine operating state is such that the optimum value of the valve overlap amount does not change much at all with a retard-correction of the ignition timing, resulting in the valve overlap amount not deviating much from the optimum value, the valve overlap amount, which has a value near the optimum value, may be less than the upper limit guard value set as described above even if the retard-correction is performed. One example of this kind of engine operating state is one in which engine output should be given priority. In this case, because the valve overlap amount is not reduced by the upper limit guard value, the valve overlap amount is not made smaller than is necessary to suppress the internal EGR amount from becoming excessive.
The guard controller may also set the upper limit guard value lower as the correction amount becomes a value that increasingly retards the ignition timing.
Accordingly, it is possible to continuously change the upper limit guard value according to the correction amount for the retard-correction of the ignition timing. Further, it is possible to smoothly change the valve overlap amount when the upper limit guard value changes as described above while the upper limit guard is applied so as to limit the valve overlap amount using the upper limit guard value.
The guard controller may also not apply the upper limit guard to limit the valve overlap amount using the upper limit guard value until the correction amount becomes a value which retard-corrects the ignition timing a predetermined amount or more.
When the retard-correction of the ignition timing to suppress knock is small, the deviance of the valve overlap amount from the optimum value due to that retard-correction of the ignition timing is also small so the deviation of the valve overlap amount from the optimum value at this time does not pose a problem. According to this construction, the upper limit guard is not applied so as to limit the valve overlap amount using the upper limit guard value in this case. Accordingly, the valve overlap amount is not needlessly reduced by the upper limit guard, which makes it possible to inhibit the internal EGR amount from being reduced more than is necessary.
The guard controller may also set a minimum value for the upper limit guard value according to the operating state of the engine.
According to this construction, because the minimum value for the upper limit guard value is set according to the engine operating state, the upper limit guard value is not reduced lower than is necessary. Therefore, it is possible to inhibit the valve overlap amount from being made smaller than is necessary when the upper limit guard is applied to limit the valve overlap amount using the upper limit guard value.
The guard controller may also not apply the upper limit guard to limit the valve overlap amount using the upper limit guard value when the engine is operating under a high load where engine output is to be given priority.
In the high load operating range of the engine where priority is given to engine output, the amount of intake air required by the internal combustion engine is large, so the valve overlap amount is adjusted so as to obtain this required amount of intake air. If the upper limit guard were applied at this time so as to limit the valve overlap amount to a small value using the upper limit guard value, the necessary intake air amount might not be able to be obtained, in which case the output performance of the engine would decease. Also, when the intake air amount of the internal combustion engine is large, as described above, the amount of internal EGR resulting from the valve overlap amount is small, so it is not necessary to restrict the valve overlap amount to a small value by the upper limit guard value in order to suppress the internal EGR amount from becoming excessive following a retard-correction of the ignition timing. According to this construction, because the upper limit guard is not applied to limit the valve overlap amount when the engine is in a high load operating state where priority is given to engine output, it is possible to inhibit the valve overlap amount from being needlessly reduced and limited to a small value, and thus inhibit the output performance of the internal combustion engine from being decreased.
Further, the guard controller may, when applying the upper limit guard to limit the valve overlap amount using the upper limit guard value, reduce the valve overlap amount by retarding the intake valve and advancing the exhaust valve, and vary a ratio of a retard amount of the intake valve to an advance amount of the exhaust valve according to the engine operating state.
According to this construction, when the valve overlap amount is restricted to a small value by the upper limit guard, the ratio of the retard amount of the intake valve and the advance amount of the exhaust valve that is necessary to realize the reduction in the valve overlap amount can be set appropriately according to the operating state of the engine. Therefore, the reduction in the valve overlap amount can be performed appropriately according to the operating state of the engine and the deterioration of fuel efficiency and torque fluctuation following that reduction can be suppressed to the minimum.
The guard controller may also vary, in accordance with the engine operating state, the rate at which the valve overlap amount is reduced when reducing the valve overlap amount with the upper limit guard value.
Accordingly, when the valve overlap amount is limited to a small value by the upper limit guard, the rate at which the valve overlap amount is reduced can be set appropriately according to the engine operating state. Therefore, the reduction of the valve overlap amount is able to be performed appropriately according to the engine operating state.
The guard controller may also set the upper limit guard value based on an engine load and the correction amount by referencing a map.
By setting the map so that the upper limit guard value when the engine is operating under a high load is a value which corresponds to a valve overlap amount that is suitable for that engine operating state, it is possible to not have the upper limit guard be applied to limit the valve overlap amount when the engine is operating under a high load. Further, by setting the map so that the upper limit guard value becomes a value which corresponds to a valve overlap amount that is suitable for that engine operating state, it is possible to not have the upper limit guard be applied to limit the valve overlap amount until the correction amount becomes a value that will retard-correcting the ignition timing a predetermined amount or more. Further, by setting the map so that the upper limit guard value becomes smaller as the correction amount becomes a value which increasingly retards the ignition timing, it is possibly to continuously change the upper limit guard value in response to that change in the correction amount. Accordingly, by setting the map so that the upper limit guard value described above is obtained according to the engine load and correction amount, it is possible to obtain an upper limit guard value that is suitable for each of the above-described conditions using only one map.
A second aspect of the invention relates to a control method for an internal combustion engine, which includes the steps of suppressing knock by retard-correcting an ignition timing based on a correction amount which is increased or decreased depending on the presence or absence of knock, controlling a valve overlap amount of an intake valve and an exhaust valve to a value suitable for an engine operating state, and applying an upper limit guard to limit the valve overlap amount using an upper limit guard value in accordance with the correction amount and the engine operating state.